Let's build the sentence, a bit at a time, to make sense of it. We start with the "core" of the sentence. 1 Some fact disappoints me.
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Amir OghlowWhy is "which is quite surprising" non-defining?Because you can remove it without loss of any key information.
Amir OghlowWhy is "which is quite surprising" non-defining?It has no definite antecedent. In particular, it is not the lecture which is quite surprising. It's the whole situation in which the lecture is cancelled that is surprising.